More details here:
http://ior.rml.co.uk/issue2/rd/universities/bath.htm
$this->bbcode_second_pass_quote('', 'T')HAI - 'Toe-to-Heel Air Injection', is a radically new process, which remedies the problems associated with conventional in-situ combustion (ISC). Conventional ISC operates as a long-distance displacement process (Figure1), due to the conventional well-spacing arrangement used. THAI is an integrated horizontal wells process, and operates via short-distance displacement, as shown in Figure 1. It therefore falls into the same category of heavy oil processes as SAGD (Steam Assisted Gravity Drainage) and VAPEX (Vapor Extension, of SAGD). In THAI, heavy oil, which is mobilized in the Mobile Oil Zone (MOZ), ahead of the combustion front (Figure1), drains into the open section of the horizontal producer a short-distance below. Typically, this is only a few tens of metres, in contrast to 100's of metres for conventional, long-distance displacement.
THAI operates in a HTO mode (high temperature oxidation), typically 450 to 600 º for a heavy oil reservoir. This is absolutely essential to maintain vigorous in-situ combustion, and avoidance of LTO (low temperature oxidation). Over 50 three-dimensional combustion cell experiments, at low pressure, have been performed by the Bath IOR Group. A typical set of results for Athabasca Tar Sand bitumen is shown in Figure 1. The THAI process can be applied to heavy or medium heavy oil (e.g. West of Shetlands Clair oil demonstrated very good combustion characteristics), and can be operated in primary or secondary/tertiary mode (post-waterflood, - cold production, -steamflood). The process is very stable, due to the near-vertical combustion front, as shown by the STARS numerical simulation for Wolf Lake Oil in Figure 2. Oxygen breakthrough into the horizontal producer well is prevented by the (mainly) forced flow displacement in the MOZ, which acts to control gas override. Residual fuel (coke) deposited in the lower part of oil layer creates a 'sealing' or permeability profiling effect, directing injected air towards the high temperature combustion zone, in the upper part of the oil layer. The principal benefits of THAI are: very high oil recovery, 80-85% OOIP; substantial in-situ upgrading (6-8 ºAPI) - worth 40 ¢ for every API point); high thermal efficiency and substantial environmental benefits, including reduction of S (30-40%),N (>90%),heavy metals (>90%). The process also generates power station amounts of energy in the reservoir. If recovered, it could provide most of the energy to run upstream operations and surface facilities, and contribute towards creating a sustainable IOR process.
$this->bbcode_second_pass_quote('', 'C')APRI: this process is a catalytic extension of THAI. A standard refinery hydrotreating catalyst (NiMo/CoMo) is employed, gravel-packed, around the horizontal producer well (Figure 2). Lighter oil in the MOZ (thermally cracked) is displaced and caused to flow over the annular layer of catalyst and into the horizontal producer well. CAPRI achieves an 8 ºAPI upgrading, additional to that achieved by THAI. There is the potential, therefore, to in-situ upgrade a heavy oil or bitumen of 8-10 ºAPI gravity, virtually to a light oil of 24-26 ºAPI (Figure 2), in a one-step downhole process. This downhole technology has potential to make huge savings on the cost of surface upgrading plant by using the reservoir as a downhole reactor.